1. Field of the Invention
The invention relates to a device for operating a short arc discharge lamp. The invention relates especially to a device for operating an ultra-high pressure mercury lamp in which an arc tube is filled with at least 0.15 mg/mm3 of mercury and in which the mercury vapor pressure during operation is at least 110 atm, and which is used for a light source for a projector device, such as a liquid crystal display device or a DLP (digital light processor using a DMD (digital micro mirror device) or the like.
2. Description of the Related Art
In a projection-type display device, there is a demand for illumination of images onto a rectangular screen in a uniform manner and with adequate color rendering. Therefore, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps, and more and more often point light sources are being produced and metal halide lamps with extremely small distances between the electrodes are being used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 150 atm, have been recently proposed. Here, the broadening of the arc is suppressed (the arc is compressed) by the increase of the mercury vapor pressure and a great increase of light intensity is the goal.
These ultra-high pressure discharge lamps are disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (corresponding to U.S. Pat. No. 5,109,181), Japanese patent disclosure document HEI 6-52830 (corresponding to U.S. Pat. No. 5,497,049), and Japanese patent specification 2980882.
For the above described lamp, for example, an ultra-high pressure mercury lamp is used in which, in the silica glass arc tube, there is a pair of opposed electrodes that are separated from each other by a distance of at most 2 mm and in which this arc tube is filled with at least 0.15 mg/mm3 of mercury and halogen in the range from 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3. The main purpose of adding the halogen is to prevent devitrification of the arc tube. However, in this way, the so-called halogen cycle also arises.
In the above described ultra-high pressure mercury lamp (hereinafter also called only a “discharge lamp”) there are cases in which, in the course of operation, the tungsten comprising the electrodes is transported to the inner surface of the discharge vessel and that so-called blackening of the tube wall occurs in which tungsten adheres to the inner surface of the discharge vessel. This blackening of the tube wall causes a reduction of the light intensity and furthermore breaking of the discharge vessel by increasing the thermal load on the tube wall in the case to considerable blackening of the tube wall; this leads to a shortening of the service life of the lamp.